Metathesis is a catalytic reaction and involves the interchange of alkylidene units among olefinic hydrocarbons via the formation and cleavage of carbon-carbon double bonds. The metathesis reaction may occur between two of the same type of molecules, referred to as self-metathesis, and/or may occur between two dissimilar types of molecules, referred to as cross-metathesis. Metathesis is a well-known and useful synthetic step in the production of industrial chemicals. Metathesis reactions are typically catalyzed by transition metal carbene complexes, for example, complexes comprising ruthenium, molybdenum, osmium, chromium, rhenium, or tungsten.
When metathesis is performed with polyunsaturated hydrocarbons as starting material, added costs and complexity are introduced into the reaction. Each carbon-carbon double bond in the system is a potential reaction site for the metathesis catalyst and also a potential site for catalyst deactivation. Unneeded sites of reaction increase the catalyst demand and can increase the complexity of the reaction product mixture. This is especially apparent in the self-metathesis of polyunsaturated fatty acid esters, where the metathesis product is a complex mixture of multiple diesters, monoesters, and internal olefins.
A specific complication arises in the self-metathesis of naturally occurring oils comprising polyunsaturated species (e.g., polyunsaturated polyol esters, polyunsaturated fatty acids, or polyunsaturated free fatty esters). The naturally occurring methylene interrupted cis, cis configuration that is prevalent in most of these oils can form 6-carbon structures, for example, cyclohexadienes having the carbon-carbon double bonds at various locations in the ring. These molecules represent volatile organic components (VOC) as part of the product, which leads to a loss in yield and a potential safety hazard. Therefore, it would be beneficial to selectively reduce the number of double bonds in the polyunsaturated compositions to compositions containing monounsaturated species in order to achieve better catalyst efficiency, reduce VOC production, and attenuate product losses.